The present invention relates to a hub unit incorporated in the drive wheel side of a vehicle, etc.
Since it is necessary that a power transmission device which transmits the power of engine of an automobile, etc., to wheels matches angular and axial displacement resulting from changes in the relative positional relationship between engine and wheels, for example, one end of a drive shaft intervening between the engine side and the drive wheel side is connected to differential gears via a plunging type constant velocity universal joint while the other end is connected to drive wheel via a hub unit including a fixed type constant velocity universal joint. The hub unit comprises an axle hub connected to drive wheel along with brake rotor, an axle bearing which rotatably supports the axle hub with respect to body side supporting member (knuckle portion, etc.), and a constant velocity universal joint for transmitting power of the drive shaft to the axle hub. As a constant velocity universal joint of the hub unit, such a structure (ball fixed joint) has generally been known, in which torque transmitting balls are, respectively, disposed in six ball tracks opened to be like a wedge in one axial direction.
As regards the hub unit, such a structure (hereinafter called a "compact type hub unit") has been developed, in which an axle hub, axle bearing and constant velocity universal joint are assembled with each other, and various types thereof have been proposed, in view of decreasing the production costs, number of assembling steps, lightening the weight, widening the operating angle in line with movements of the joint center, and improving the steering performance. However, conventional compact type hub units have problems as described below, which become obstacles in practical applications.
(1) The design of tire size, wheel rim and brake rotor is determined in order, depending on the output of engine, and the diameter of a shaft portion of the constant velocity universal joint is also determined on the basis of the engine output. The compact type hub unit is constructed so that the axle bearing intervenes between the brake rotor and an outer joint member (also acting as the axle hub) of the constant velocity universal joint. Actually however, since the outer diameter of the axle bearing is made too large, it is difficult to accommodate the hub unit at the bore side of the brake rotor.
(2) Since the axle bearing and the constant velocity universal joint are very close to each other, the internal exothermic reaction of the constant velocity universal joint is transmitted to the axle bearing, and there is a possibility that the service life of the bearing is decreased.
(3) Since on the bore side of the brake rotor the empty space is small heat collects therein resulting in a temperature rise of the axle bearing and the exothermic reaction of the brake.
(4) In comparison with a case where the respective members are independently disposed, the total weight is decreased. However, the hub unit itself is made heavy and its scale is increased. Thus, it is difficult to handle the hub unit.